1. Field of the Invention
The present invention relates to a quadrature amplitude modulation (QAM) signal generator.
2. Description of the Related Arts
Various multilevel optical modulation formats such as DQPSK (Differential-Quadrature Phase-Shift-Keying), QPSK (Quadrature Phase-Shift-Keying) in a coherent detecting system, APSK (Amplitude-and-Phase-Shift-Keying), and QAM (Quadrature-Amplitude-Modulation) have been developed for the purpose of improving the frequency utilization efficiency of channels for optical communications.
Especially, DQPSK using dual parallel Mach-Zehnder modulators (DPMZM) is the most practicable and has received particular attention, since it can modulate the in-phase components (I components) of a carrier signal and the quadrature phase components (Q components) of the carrier signal independently by using the modulators. There has been developed a DQPSK signal generator that can generate an optical modulation signal over 100 Gb/s owing to two high-speed binary data signal systems having a modulation speed of 50 Gbaud or more.
The QAM scheme has higher spectral efficiency compared to the QPSK scheme. Employing DQPSK using the above-described DPMZM can improve the performance of the QAM scheme. However, such technology has a problem that it is not easy to handle multilevel electrical signals.
Input binary data streams are conventionally used by converting them into multilevel electrical signals. At present, high-speed arbitrary waveform shaping devices are generally used for that purpose. For electrical circuits, nonlinearity is being suppressed, and the circuit frequency response is being designed carefully. For such reasons, the rate of QAM signals is only a few Gbaud or less.
Therefore, there has been needed a device which can obtain optical signals such as QAM signals without handling multilevel electrical signals, and a method therefor.